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A law enforcement source says Dallas shootings suspect Micah Xavier Johnson told police he had been training for the attack – working out – in preparation for a mass murder of police officers. (Published Friday, July 8, 2016)

A North Texas Army veteran has been identified as the lone gunman responsible for the sniper attacks that killed five police officers and injured seven others in Dallas, authorities say.

The investigation into Johnson's attack is still ongoing, and much remains is still unknown. But a picture is beginning to emerge of what went on inside the standoff — a source tells NBC Investigates that the 25-year-old was wounded by gunfire before being killed by a robot outfitted with a bomb — and how he prepared for the deadly assault.

"This was a mobile shooter that had written manifestos on how to shoot and move, shoot and move, and he did that. He did his damage," Rawlings said.

Officials told NBC News the investigation so far has yielded no additional suspects that may have played a role in the shooting. Texas Gov. Greg Abbott said Friday that there is no information about additional co-conspirators, but if any are found, they will be brought to justice.

Sources tell NBC News they have found no ties between Johnson and any extremist groups so far.

"We believe now, that the city is safe," Rawlings said. "The suspect is dead, and we can move on to healing."

INSIDE THE STANDOFFJohnson was laughing and singing and not at all anxious during the standoff at the El Centro College building, a law enforcement source with knowledge of the incident told NBC 5 Investigates senior reporter Scott Friedman.

Johnson told police he had specifically been training for this event and working out in preparation for Thursday night. NBC 5 Investigates has also learned Johnson was wearing a military-style bulletproof vest.

We believe now, that the city is safe. The suspect is dead and we can move on to healing.

Dallas Mayor Mike Rawlings

Johnson told police he spent time in the military and was carrying a military-style rifle.

Johnson was hit by gunfire before going into the El Centro college building and that officers followed Johnson's blood trail into the building, according to a law enforcement source.

Officers found him on the second floor, and then fired more rounds through a wall, apparently hitting Johnson again and wounding him.

After that, the negotiations began and spanned several hours. Johnson threatened many times to charge the officers, according to the source.

Johnson at first said that he only wanted to talk to black police officers – he said he didn't want to have anything to do with white people. He shared police conspiracies and his dislike for police officers, a law enforcement source said.

Officers cornered Johnson and negotiated with him for hours before talks broke down, police said.